Too much fuel pressure

Finally back to solve my fuel flow issue. I put a new fuel pump on my 69 XR7 and discovered that the fuel pressure was way too high. Couldn’t figure out why a new pump would have so much more pressure but solved it quickly by adding a regulator to lower the pressure. I just had to put a new fuel pump on my sons 78 F350 truck and the same thing happened — so I am back trying to permanently solve the issue on the cougar. I don’t like having a regulator on it. But the engine won’t even start without the regulator.
So why does a new fuel pump produce so much more pressure??

Is it a replacement mechanical unit? Bolts to the same place on the engine as the factory one? I have had that problem with electric pumps. I just put a new one on my car this last spring and have no problems with it. (On a 1970 351W)

What fuel pump did you install on your car?

A stock mechanical fuel pump for a carburated engine should put out only 6 psi. If you are running a high performance mechanical fuel pump such as a holley 12-289-35, it will put out almost 16 psi which will blow the needle valve open so yes you will need to run a regulator on this type of pump. Most electric fuel pumps (especially designed for fuel injected systems) are high pressure and must be regulated as well (assuming you are using a carburetor).

Coach Jack

Yes, it is a replacement mechanical pump - just picked it up from O’Reillys. And an Edelbrock carb. I even have the regulator adjusted as low as I can and it still seems to be getting too much fuel. Starts right up when cold. But after warmed up and shut off for 10 or 15 minutes I have to treat it like its flooded in order to get it going again (hold the accelerator to the floor until it fires). And when it does start after that, I get black smoke out of the tail pipes. I just thought I could bolt on a new pump and go!

How much pressure is the new pump pumping?

Also since you stated it starts fine when cold but acts as if its flooded after 10-15 minutes, check to make sure the choke on the carb is operating correctly. Sorry I don’t have an edelbrock carb to help.

Coach Jack

Be sure that the choke is coming off. Try blocking it open and see if the symptoms change.

Thanks, I’ll give it a try … But I still wonder – I have researched and asked a lot of folks and can’t get a good answer. Why does a new mechanical fuel pump purchased at O’Reillys seem to pump so much more fuel than the one I took off? Am I missing something?

If it happened to just one car, I would assume I am the problem. But two vehicles - same problem. I must be missing something here. I would love to know.

New pumps often have junk inside them from the manufacturing process that needs to be flushed out before installation. Disconnect the fuel line from your carb and run tha gas into a coffee can for a bit to make sure it’s clean.

This isn’t an indication of too much fuel pressure. People tend to think that a carb squirts fuel under pressure into a stream of air. That is not the case at all. Air passing through the carb creates a low pressure area that sucks fuel through openings in the air stream. The fuel pump is there to replenish the level of fuel in the bowl. The level is regulated by a float. When the fuel level rises, the float pushes up against a needle that seals the fuel inlet. It only allows more fuel to enter when the level drops.

The symptom of too much fuel pressure is that the bowls over flow. The pressure is too high and it pushes the needle off the seat, pushing the float down into the fuel. You will see fuel over flowing from the carb coming out of the vents. The engine will stall or catch on fire if your luck runs out.

The problem you are describing is caused when the fuel in the bowl overheats after you have parked the car. The fuel boils and the expansion pushes raw fuel out of the carb into the intake manifold. When you go to start the car it is overly rich, meaning you have to compensate by increasing the amount of air available (holding the throttle to the floor) and when it does start it will put out black smoke which is un-burned fuel. At the same time your fuel pump is trying to refill the bowls and so the incoming mixture may be a bit lean for a while. All in all it makes it hard to get it started and to keep it running.

I have had this issue with Edelbrocks before. The fuel bowls sit right on top of the intake manifold unlike a Holley where the bowls are surrounded with air. It can also happen with a Holley but it seems less frequent. The Edelbrock as it comes out of the box is set up for a 350 Chevy. Due to the differences in the way timing is set up, they tend to be too rich for small block Fords. My guess is that you are running too rich to begin with and that contributes to running hotter than usual. I would get the Edelbrock tuning chart and change the needles and seats needed to move towards lean by about two steps. You can tell if have gone too far if it develops lean surge while cruising at about 50 MPH on level ground.

To add to what others have said, did you actually measure the fuel pressure?

This is really helpful. Best explanation I’ve heard. Thank you so much. It makes sense that blocking the choke open might help the problem but may be only a superficial fix. The real issue is to lean out the carb. Thank you. And, yes, Vert the pressure was registering just over 8psi - the pump was advertised at 6-8. I have adjusted the regulator to 2psi and the problem persists so I will try disconnecting the choke to see if that changes things. Then I will get out the carb charts as g428 suggests. That seems like the best and permanent solution. I hope you save that explanation. I know a few mechanics who could learn from it!


Even the Edlebrock site states that the fuel pressure should not exceed 6.0 psi and optimum is 5.5 psi. Oreily’s sells a few different mechanical fuel pumps and their site shows a max 6.0 psi for a 302 and also another that has a maximum of 8.0 psi, but again, this exceeds the Edelbrock specifications. BTW, what size engine are you running and what size Edelbrock Carb did you install (as this will also affect your operating parameters)?

Coach Jack